1. Field of the Invention
The invention is generally related to the inspection of nuclear fuel assemblies and more particularly to the detection of leaking fuel assemblies.
2. General Background
During nuclear reactor operation, gaseous fission products are formed inside the fuel rods, which are pressurized with an inert gas during manufacture. If a breach occurs in the fuel rod cladding, the fission gases may escape. It is very important to nuclear utility operators that all leaking fuel be identified, so that the leaking fuel can be repaired or removed from service. Leaking fuel results in the release of fission products into the plant coolant system, which causes increased radiation exposure to plant personnel and equipment, and may cause expensive cleanup operations. On-line sipping is a method used to detect leaking fuel assemblies in nuclear power plants. The on-line sipping system is designed to detect these gases during fuel offload from the reactor core. As the assembly is lifted from the core and into the fuel handling mast, the drop in external hydrostatic pressure will allow the fission gases to escape. On-line sipping differs from other testing methods where the fuel assembly is removed from the reactor and placed in a closed container where it is then necessary to drive the gas from any failed fuel rods by the use of heat or a vacuum. A typical on-line sipping system utilizes two or four nozzles to inject air under the fuel assembly while it is being lifted into the fuel handling mast. The air helps to move the fission gases to the surface of the water, where the air/gas mixture is vacuumed off and passed by a gamma radiation detector which identifies gamma-emitting fission products. The air injection system typically utilizes an air compressor or plant compressed air supply, air lines, and two or four injection nozzles to direct air horizontally under the fuel assembly during inspection. The nozzles are sized to produce small diameter air bubbles that should cover the entire bottom surface of the fuel assembly and travel upward throughout the entire assembly. These bubbles, as they travel up through the assembly to the water surface, will capture fission gases that are held to the fuel assembly by surface tension. These bubbles also create water turbulence which helps to drive fission gases to the water surface. It has been observed, that even with various sized air injection nozzles, air flows, and pressures, complete coverage of the bottom of the fuel assembly is not feasible. This is due to the lack of momentum of the air bubbles and the resistance of the water. Rather than spreading to cover the entire fuel assembly, the air is only delivered to the local areas where the nozzles are located, and as the air bubbles travel upward through the fuel assembly, they do not spread out through the assembly. This reduces the efficiency of the detection and could cause a leaking fuel assembly to escape detection. In an effort to improve the bubble distribution, some suppliers of on-line sipping systems have utilized four nozzles rather than two. This approach delivers air to each corner of the assembly but still does not approach complete coverage. Another problem with the conventional method is the high potential for the nozzles to become partially or completely obstructed , further reducing the efficiency of fission gas detection. Even small debris within the supply air lines can become lodged in the nozzles because the nozzle openings are normally very small.